Devices for injecting medication beneath the skin without the use of a needle by forcing the medication through the epidermis in a high pressure jet are well known to the art. For this purpose it is known to the art to employ an ampul which contains a plunger to force the liquid from the ampul. The prior art devices have a ram for advancing the plunger. Such rams are either spring or gas actuated. Prior art patents are U.S. Pat. Nos. 3,292,622; 3,688,765 and 3,292,621. As against the devices having a spring actuated ram, the devices with a gas actuated ram greatly facilitate providing a convenient design since it eliminates the inflexibility of having to place a powerful compression spring in line with the ram. The most successful gas actuated devices have suffered from being erratic in the injection force generated due to the variation in gas pressure arising from temperature changes arising, for example, from the use of a liquified gas which exerts a cooling effect on vaporizing. Difficulty has been encountered incident to the use of a mechanical system to hold the ram and release it at a predetermined pressure. In addition, gas is readily wasted thus reducing the number of shots possible from the gas source when a gas cartridge is employed. The injection device of this invention solves these problems.